A variety of spinal conditions result in a person experiencing pain or limited physical activity and ability. More specifically, damage to vertebrae composing the spine and spinal discs between the vertebrae may occur as a result of trauma, deformity, disease, or other degenerative conditions. Some of these conditions can be life-threatening, while others cause impingement on the spinal cord resulting in pain and a lack of mobility. Removing the impingement, thus reducing swelling or pressure from the damaged or diseased tissue against the spinal cord, can relieve the pain and often promotes healing and return of normal nervous system functioning. However, the absence of proper medical care may lead to further damage and degeneration of spinal health and to permanent spinal cord damage.
The spine principally includes a series of vertebrae and spinal discs located in a space between adjacent vertebrae. The vertebrae are formed of hard bone while the discs comprise a comparatively soft annulus and nucleus. The discs support the vertebrae in proper position and enable the torso to be rotated and to bend laterally and anteriorly-posteriorly. The discs also act as shock absorbers or cushions when the spine is experiencing shock, such as when a person jogs.
Damage to the spine often results in a reduced physiological capability. For instance, damage to the disc may allow the annulus to bulge, commonly referred to as a  herniated disc. In more severe cases, the damage may allow the nucleus to leak from the annulus. These same results may be brought about by a damaged or fractured vertebra. In any event, such damage often causes the vertebrae to shift closer or compress, and often causes a portion of the disc to press against the spinal cord.
One manner of treating these conditions is through immobilization of the vertebrae in a portion of the spine, such as two or more adjacent vertebrae. The conditions often lead to degeneration and a loss of disc support, and immobilization is often beneficial in reducing or eliminating pain. Immobilization and/or fusion have been performed via a number of techniques and devices, and the type of injury often suggests a preferred treatment regime.
One of these treatments is known as spinal fusion surgery. For this, two or more adjacent or consecutive vertebrae are initially immobilized relative to each other and, over time, become fused in a desired spatial relationship. The vertebrae are relatively immobilized at the proper intervertebral distance which replicates the support characteristics of the spine. This prescription sacrifices the rotation or flexion between the affected vertebrae, such that some loss of movement and flexibility is experienced. However, the compression on the spinal cord due to the injury or damage is reduced or eliminated, and the fused vertebrae protect the spine and spinal cord from injury. Overall, the non-fused portions of the spine are largely able to compensate for most normal movement expected by a patient.
Currently, a number of vertebral body replacement devices (VBRs) for immobilizing and fusing adjacent vertebrae are known. During an implantation procedure, the intervertebral space is initially excavated to provide a volume for locating a VBR therein. Once excavated, the adjacent vertebrae have a tendency to shift toward each other a small amount, thereby compressing the space or volume. Additionally, many VBRs have surface features such as prongs or teeth which extend away from upper and lower surfaces of the VBR for being embedded into the adjacent vertebrae. In order to locate the device within the intervertebral space, instruments may be used to spread the vertebrae apart. During such a procedure, care must be taken not to damage the spinal cord. The VBRs may then be inserted into the intervertebral space in an orientation where the surfaces with teeth thereon face the adjacent vertebral surfaces. However, if the vertebrae are not sufficiently distracted, VBR  insertion can be difficult due to resistance generated when the teeth engage the vertebrae, particularly if the implant needs to be redirected or turned in the intervertebral space to an implantation orientation that is offset from the insertion orientation thereof.
Accordingly, it has been disclosed that the VBR may be inserted initially into the intervertebral space in a first orientation where the upper and lower surfaces and the teeth thereon face laterally outward and then be rotated secondarily so that the teeth are brought into engagement with and embed into endplates of the vertebrae. This allows distraction of the adjacent vertebrae to be kept to a minimum. Stability of the spine benefits from the VBR having a contour or shape that generally follows the surface shape of the endplates. As the endplates are generally slightly concave, the surface portions of the VBR including the gripping teeth often have a corresponding contour. In this configuration, the spacing between the side surfaces of the VBR will generally be less than between the toothed surfaces for maintaining the vertical distraction required prior to VBR insertion.
Accordingly, rotation of the VBR in the intervertebral space may result in significant stress upon the VBR since the rotation may require forcing the vertebrae apart a small amount. Because the VBR typically has a body of relatively small size for fitting in the intervertebral space and particularly if cavities are formed therein for graft material, VBR rotation can generate undesirably high compressive force on the VBR.
A number of solutions have been attempted for addressing these compressive forces due to the rotation. The trend in the field is that device manufactures are reducing the size of VBRs to such a degree so that rotation thereof requires less separation of the vertebrae. However, this solution comes at the expense of having the VBR securely positioned in the intervertebral space with the teeth securely gripping the adjacent vertebral surfaces. Another trend is to form the VBRs with a strong material, such as PEEK, and avoid the use of natural bone or artificial bone materials such as hydroxyapatite or allograft.
Unfortunately, these stronger materials are not bio-resorbable. The purpose of the fusion procedure is to develop a lattice, matrix, or solid mass of bone joined with and extending between the adjacent vertebrae and through the intervertebral space. Eventually, the formed or developed bone and the vertebrae are joined to provide a somewhat unitary,  incompressible structure that maintains the proper pre-fusion spatial relationship for the size to reduce or eliminate the impingement on the spinal cord. The VBR formed of these stronger materials is unable to transubstantiate into bone, join with bone, or be absorbed by the body for replacement by bone growth. This results in a boundary interface between the implant device and any resultant bone growth. Again, this is often addressed by reducing the size of the VBR so that more graft material may often be packed into the intervertebral space around the VBR. However, as mentioned, this is done at the cost of having secure implantation of the VBR.
As noted, the intervertebral space receives the VBR or implant device as well as an amount of graft material. The graft material may be in a number of forms, such as cancellous bone chips, which are packed into the intervertebral space and around the VBR. For VBRs with internal cavities opening on at least one side to the intervertebral space, graft material is also placed within the cavities so that bone may grow through the VBR device and join with bone formation throughout the intervertebral space.
However, as these bone chips are loose and oftentimes fragile, migration of the bone chips from the intervertebral space presents an issue. While implanting more bone graft material promotes faster bone formation throughout the intervertebral space, the loose bone chips or graft material portions tend to separate from each other, a tendency which is exacerbated by being more tightly packed. Full fusion may take upwards of two years, during which time a patient's movement may contribute to the graft material explanting from the intervertebral site. In general, previous solutions to this problem have consisted of sewing the intervertebral site closed, such as by retaining and re-closing the natural damaged annulus, or by providing the cavities within a VBR.
Another issue confronted with the implantation of the VBR is in situ adjustments or positioning. Once rotated, the teeth are engaged with the endplates thereby making adjustment difficult. If extensive adjustments are made in situ, the teeth may erode or carve out additional space which reduces the distance between the vertebrae. If a minimal proper distance is not maintained, the spinal cord may still be impinged. Often such impingement is  difficult to recognize until after the fusion has progressed to a point where revision surgery is difficult and complicated.
Accordingly, there has been a need for improved spinal fusion systems and for improved methods for performing spinal fusion surgery.